RETRIEVAL SYSTEM, RETRIEVAL METHOD AND RETRIEVAL PROGRAM

PROBLEM TO BE SOLVED: To detect illegalities in a server (deletion, addition, alteration and replacement of a file), and efficiently perform update (including deletion) of the file.SOLUTION: A user terminal of a retrieval system storing a plurality of files in a server, acquiring a collection of desired files from the server by keyword retrieval and performing update of the files comprises an index verification information generation unit which generates index verification information according to a prescribed generation algorithm for a set for generation composed of a keyword and a collection of file numbers of all files including the keyword, and stores the information in the server. The user terminal transmits a retrieval keyword to the server, and acquires the collection of the file numbers of files including the retrieval keyword and the index verification information corresponding to the retrieval keyword from the server. An index verification unit checks whether or not the acquired index verification information is the index verification information of a set for verification composed of the retrieved keyword and the acquired collection of the file numbers of files.